1. Field of the Invention
The present invention generally relates to an image processing circuit and an image processing method thereof, and more particularly, to an image processing circuit and an image processing method thereof for performing adaptive processing according to the image characteristic.
2. Description of Related Art
When watching images, a viewer pays attention to not only the color and the hue of the images, but also to the profiles or the edges of the objects in the images. The image processing is used to enhance the image features for improving or increasing the visual effect of the images. For example, images obtained by a digital camera or a scanner usually are processed with a post procedure so as to look more clear and rich-colorful.
One of the image processing techniques is sharpening processing, wherein in an image undergone the sharpening processing, the profiles or the edges of the image objects have higher contrast to distinguish the image object from others. On the other hand, ‘sharpness’ usually represents the sharpened extent of an image. For a viewer, an image with higher sharpness usually looks more clear than that with lower sharpness.
Although the sharpening processing can gain a higher contrast of the profiles or the edges of the image objects, but it may cause negative effect as well. For an image where the edges of the objects are clear already, a further sharpening processing may make the edges thereof generate unacceptable visual effect such as light contour.
US Patent Application No. 2005/0270425 discloses a video signal processing device 100, as shown by FIG. 1. Referring to FIG. 1, a video signal processing device 100 receives an input video signal SA, performs a sharpening processing on the input video signal SA and then produces an output video signal SD. FIGS. 2A-2E are timing charts of the partial signals in the video signal processing device 100 of FIG. 1. Referring to FIG. 2A, the input video signal SA herein includes a general video signal S and noises n1 and n2, wherein the general video signal S carries the image information. The video signal processing device 100 would increase the sharpness of the images in the general video signal S and avoid the noises n1 and n2 from being sharpened.
The video signal processing device 100 includes a signal delayer 101, a weighing unit 103, a first multiplier 109, a second multiplier 111, an adder 113 and a high-pass filter 115. Referring to FIGS. 1 and 2B, the high-pass filter 115 receives the input video signal SA and performs a high-pass filtering processing on the input video signal SA so as to produce a high-frequency signal SB. The high-frequency signal SB is used as base for judging the profiles/edges of the image objects. In more details, the amplitudes of the video signal S are corresponding to the pixel values of the frame pixels. When the amplitudes of the video signal get varied, it indicates there are differences between the pixel values of adjacent pixels in a frame. In particular, a larger amplitude variation of the video signal S is corresponding to the portion of profiles/edges of the image objects in the frame, so that the high-pass filter 115 would filter out the component with larger amplitude variation to produce the high-frequency signal SB. As a result, the produced high-frequency signal SB would be used as base by the video signal processing device 100 for sharpening processing.
Referring to FIGS. 1 and 2C, the weighing unit 103 includes an edge calculator 105 and a weight calculator 107. The edge calculator 105 receives the input video signal SA, detects the image information in the input video signal SA and calculates the pixel differences between the present pixel and the adjacent pixels thereof so as to judge which area the present pixel belongs to. After that, the weight calculator 107 accordingly produces a weighing signal S1 according to the judgement result of the edge calculator 105, wherein the weight value contained by the weighing signal S1 would be accordingly varied depending on different areas the pixels belong to.
A gain signal S2 is input to the first multiplier 109 for determining the gain for the sharpening processing performed by the video signal processing device 100. The first multiplier 109 multiplies the weighing signal S1 by the gain signal S2 to produce a total gain signal S3. Referring to FIGS. 1 and 2D, the second multiplier 111 multiplies the total gain signal S3 by the high-frequency signal SB to produce a peaked signal SC. Meanwhile, the signal delayer 101 delays the input video signal SA so as to output a delayed video signal S4 to the adder 113. Referring to FIGS. 1 and 2E, the adder 113 adds the delayed video signal S4 to the peaked signal SC so as to accomplish a sharpening processing and produce the output video signal SD, wherein the output video signal SD includes a video signal S′ and the noises n1 and n2, and the video signal S′ is obtained from the general video signal S after receiving a sharpening processing performed by the video signal processing device 100.
The sharpening processing method used by the video signal processing device 100 is usually termed as peaking processing method. Taking FIGS. 2A and 2E as an example, in comparison with the video signal S, the pixel values in the portions of profiles/edges of the frame objects corresponding to the video signal S′ after receiving a sharpening processing are increased or depressed; therefore, the processing method causes light contours/dark contours appearing on the frame. FIG. 3 is a localized-enlarged diagram of the video signal S′ of FIG. 2E. Referring to FIG. 3, the video signal S′ contains an area 12, and the pixel values of all the pixels corresponding to the area 12 are equal to the minimal pixel value of the frame. Assuming the output video signal SD is the video signal able to display frames with 256 gray-levels, then the minimal pixel value would be equal to zero, while, on the other hand, the maximal minimal pixel value would be equal to 255. As a result, the pixels corresponding to the area 12 of the video signal S′ produce dark contours during being displayed on a display.
In addition to the dark contours, the video signal processed by the video signal processing device 100 may also produce light contours on the display. FIG. 4 is a timing diagram of another sharpened video signal processed by the video signal processing device 100 of FIG. 1. Referring to FIG. 4, in addition to the area 12, the waveform of a sharpened video signal further contains another area 14, wherein the pixel values of all the pixels corresponding to the area 14 are equal to the maximal pixel value of the frame. As a result, the pixels corresponding to the area 14 of the video signal S′ produce light contours during being displayed on the display.